1. Field of the Invention
The present invention relates to a press machine such as, for example, a press brake.
2. Description of the Related Art
When a press brake is classified according to a drive system of a ram, two types are generally available; a hydraulic drive system utilizing a hydraulic cylinder and a motor drive system utilizing a servo motor. The motor drive system is of a type in which a revolving motion of a servomotor is translated into a linear motion by means of a motion translating mechanism such as, for example, a ball screw mechanism. In addition to those two systems referred to above, a composite drive system utilizing the servomotor and the hydraulic cylinder concurrently (such as disclosed in JP Patent No. 3558679) and a dual servomotor system utilizing two servomotors that play different roles (such as disclosed in JP Laid-open Patent Publication No. 2004-188460) have been suggested.
The hydraulic drive system and the motor drive system have their own merits and demerits. In general, the hydraulic drive system is known to be effective in providing a high power output, but difficult in controlling the speed, particularly in controlling a high speed drive. The motor drive system is known to have merits and demerits that are reverse to those of the hydraulic drive system. For this reason, according to the hydraulic drive system, a substantial amount of time is required to drive the ram from a standby position to a press start position, thus posing a problem in operating efficiency. In addition, the hydraulic drive system occasionally accompanies a leakage of oil and, therefore, it is difficult to halt the lowering of the ram completely. On the other hand, the motor drive system poses such a problem that difficulty is often encountered with in applying a high load and does therefore have limitations in use thereof, one of which includes an incapability of handling with heavy plates. Also, in view of the structure of the motion translating mechanism, the motor drive system is generally considered difficult to accomplish a speed changeover between a high speed and a low speed.
JP Patent No. 3558679 referred to above pertains to the composite drive system utilizing the servomotor and the hydraulic cylinder concurrently, in which the lack of the power output with the motor drive system is compensated for by the hydraulic cylinder. In this composite drive system, positioning of the ram is accomplished by the motor drive system and the hydraulic cylinder is merely used in an auxiliary manner. More specifically, the composite drive system is designed such that the load imposed on the ball screw mechanism of the motor drive system is first measured by a load cell so that the hydraulic cylinder can be actuated only in the event of the shortage of the power output. With this composite drive system, since pressurization is accomplished basically by the ball screw mechanism, the servomotor capable of providing a large power output needs to be selected. For this reason, the abrupt stoppage performance appears to become untolerable.
JP Laid-open Patent Publication No. 2004-188460 referred to above discloses the dual servomotor system utilizing a first servomotor for a high speed low load use and a second servomotor for a low speed high load use, which system is designed such that the servomotor for the high speed low load use is used to drive the ball screw mechanism during a period in which the ram is driven from the standby position to the press start position, but the servomotor for the low speed high load use is used to drive the ball screw mechanism during the press operation. Since this dual servomotor system is such that drive and pressurization are accomplished only by means of the ball screw mechanism, the ball screw mechanism needs to be large in size. It has, however, been found that if the ball screw mechanism is large in size, the inertias is large during a high speed drive, resulting in an increase of the stopping distance. Also, because of the pressurization accomplished by the ball screw mechanism, it is incapable of being applied to a large-size press brake.